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Alterations in Two Conserved Regions of Promoter Sequence Lead to Altered Rates of Polymerase Binding and Levels of Gene Expression

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Alterations in Two Conserved Regions of Promoter Sequence Lead to Altered Rates of Polymerase Binding and Levels of Gene Expression Number 12 1980 Volume 8 Number 12 1980 Nucleic Acids Research Alterations in two conserved regions of promoter sequence lead to altered rates of polymerase binding and levels of gene expression James E.Stefano, Jill W.Ackerson and Jay D.Gralla Biochemistry Division of the Chemistry Department and Molecular Biology Institute, University of California, Los Angeles, CA 90024, USA Received 9 April 1980 ABSTRACT Characterization of recombinant lac promoters highlights the importance of two regions of sequence conservation in promoters. The "Pribnow box" sequences are necessary for specific transcription in this system. This specificity is maintained when a mutated upstream sequence is introduced. However, changing the upstream DNA sequences influences both the rate of RNA polymerase binding in vitro and levels of expression in vivo. INTRODUCTION Two fundamental and interrelated questions concerning gene expression in E. coli are: What distinguishes promoter DNA from non-promoter DNA and what determines the maximal rate of promoter utilization? Since the answers lie ultimately in the sequence of DNA bases, much effort has been applied to determination of promoter sequences. One approach to extracting the relevant information is to compare promoters for conserved sequences which are likely candidates for functionally important promoter elements. This approach has led to the identification of two separated regions of moderate sequence conservationl16 centering near 10 base pairs ("downstream homology" or "Pribnow box") and 35 base pairs ("upstream homology") distal to the startpoint of RNA transcription (see Figure 1). A second approach is to determine the location and effects of mutations which alter the function of an individual promoter. This approach has complemented the sequence comparisons since most mutations lie either in the upstream homology region or the downstream homology regions (see Ref. 6 for a review). In vitro studies of these mutant promoters have led to various proposals ©) IRL Press Umited, 1 Falconberg Court, London W1V 5FG, U.K. 2709 Nucleic Acids Research for the role of sequences since the rate of promoter binding, level of transcription, or ability to initiate transcript may be affected by sequence changes within the two homology regions (Majors, unpublished work: ref. 7,8). In order to elucidate the role of these regions in vitro and in vivo we have constructed, using recombinant DNA techniques, double mutants of the lac operon which contain a mutation in each region of sequence homology. These double mutants and their parent strains were studied both in vitro and in vivo. The in vivo study relied on a plasmid vector which we have constructed (Ackerson et al. manuscript in preparation) to allow expression in vivo of promoters created in vitro. Downstream homology sequences are found to be necessary for significant levels of specific transcription in vitro. However, a mutation upstream, when combined with either of two downstream sequences, leads to a reduced rate of promoter binding in vitro and a reduced level of gene expression in vivo. Thus, although the downstream sequences are the principal "mark" identifying these promoters, the upstream sequences also affect levels of gene expression, probably by contributing to the rate of promoter binding by RNA polymerase. MATERIALS AND METHODS Transcription Assay of Binding Rate: Lac promoter DNA was isolated as described previously16 by endonuclease Eco Rl digestion of purified plasmid pMB9. E. coli RNA polymerase, approximately 50% active, was prepared and assayed according to the method of Burgess and Jendrisak37 and catalyzes promoter-specific transcription as judged by criteria described previously16. The conditions for transcription were essentially as described in references 16 and 19, except that acetylated bovine serum albumin35 (at 100 pg mL-1) was used. Binding rate experiments were initiated by mixing 2 picomoles of RNA polymerase and 1 picomole of purified promoter restriction fragment in a final volume of 251,L at 370 C. At the times indicated heparin was added (to 100 ig mL-1) to stop the binding reaction simultaneously with nucleoside triphosphates to 2710 Nucleic Acids Research allow RNA production from productive complexes. ATP, GTP, and UTP were at 200 UM, and a-32P-CTP at ca 15 Ci mmolV1 (prepared according to reference 38) was at 2 jM. RNA synthesis proceeded for 15 minutes at 370 C. An equal volume of saturated urea containing marker dyes was added and an alliquot loaded directly on a 12% acrylamide gel containing 7M urea34. The lac-specific RNA was located on an autoradiograph, excised, and counted for Cerenkov radiationl9* The construction of recombinant promoter strains and the cloning vehicle pAS21 is outlined briefly in the text and in detail elsewhere (in preparation). Assay of Tetracycline Resistance: Tetracycline resistance was measured by a modification of the procedure of Tait et al.23. All measurements were done in E. coli strain MM 294. A freshly diluted transformed bacterial culture was grown to an A570 of 0.5 in LB broth. Triplicate aliquots were then diluted appropriately to give 100-200 colonies per plate. Each plate contained L agar (LB broth + 1.5% agar without added glucose) and a specific concentration of tetracycline. Plates were prepared immediately before use since tetracycline potency diminishes with time. After overnight incubation at 37 C colonies were counted and the three determinations averaged. Per cent survival was calculated relative to the number of colonies appearing at the lowest concentation of tetracycline. RESULTS AND DISCUSSION The Mutant Strains: The relevant lac promoter sequences are shown in Fig. 1. Class II promoter mutations are defined as those which exhibit decreased levels of expression in vivo both in the presence (crp+, cya+) and the absence (crp , cya-) of functional catabolite activator protein9. These mutations have been sequencedlO and are base changes within the consensus upstream homology region (see L305 in Fig. 1). Class III promoter mutations allow high level expression in the absence of functional catabolite activator proteinlll2; two of these (see UV5 and ps in Fig. 1) are changes which increase homology with the consensus downstream homology region (J. Gralla, J. Majors, 2711 Nucleic Acids Research upstream downstream homology homology region region -40 -30 -20 -10 - GCTTTACA TATGTTG - CGAAATGT - ATACAAC Hpc (l A L305 I UV5 TT Figure 1. Schematic diagram of the lac promoters used. Distances marked are base pairs from the favored startpoint of the messages in vitro (shown by a wavy line). L305 is a "down" promoter mutant; UV5 and ps are "up" promoter mutants (see text). The UVS strain in addition contains the L8 CAP site mutation at position -66, not shown here. The Hae III restriction sites used to isolate the promoter fragments are located at positons +65 and -142. Conserveds romoter sequences have been pointed out by several authorslb. The wild-type sequence is from reference 15; the L305 sequence is from reference 10; see reference 3 for the UV5 and ps changes. quoted in Ref. 3). We have taken advantage of the restriction enzyme cleavage sites witnin and about the lac promoter to construct recombinant promoters containing both class II and class III mutations. Specifically, endonuclease Hae III makes two cleavages which allow isolation of an intact promoter13 while endonuclease HpaII cleaves within the promoter between the upstream and downstream homology regions. Thus, combined cleavage by these two enzymes allows isolation of an upstream half-promoter fragment of 120 base pairs and a downstream half-promoter fragment of 80 base pairs. Since individual mutant strains were available either on transducing phages14 or plasmid vectors (J. Majors, F. Fuller, unpublished), an 80 base pair fragment containing either the UV5 or ps sequence and a 120 base pair fragment containing the L305 sequence were easily isolated. Briefly, the 120 base pair L305 fragment was joined to the 80 base pair UV5 or ps fragment by T4 DNA ligase in order to obtain species which include the 200 base pair recombinant 2712 Nucleic Acids Research promoter. These products were then ligated into the Eco Rl site of the plasmid vector pMB915. Cells were transformed, plated on XG-tetracycline, and plasmids from blue colonies (an indication of cloned lac operator) were screened by restriction cleavage. A large fraction of these yielded 200 base pair inserts upon Eco Rl cleavage which when digested subsequently with Hpa II revealed the original 120 base pair fragments. At this point we had available five mutant promoter strains for study; L305, UV5, ps, L305 x UV5, L305 x ps (see Fig. 1). Each strain is cloned and the promoters contained in otherwise identical restriction fragments. Such fragments can be used in a highly purified in vitro transcription system to assay for lac m-RNA initiation (19,16,7). Figure 2 shows that the UV5 and ps promoters support specific transcription in vitro, as evidenced by the appearance of the runoff lac RNA band on the polyacrylamide gel. By contrast, the L305 DNA fails to support efficient transcription of lac RNA. Thus, the behavior of these strains in vitro mimics their behavior in vivo (see below). When either UV5 or ps was crossed with L305, the recombinants supported specific transcription (Fig. 1) as evidenced by the preferential appearance of the same length lac RNA band produced by UV5 and ps. Thus, the downstream sequences (UV5 or ps) are dominant to L305 by this limited criterion; that is, their presence allows specific transcription even in the presence of a mutated upstream homology region. Since L305 alone failed to support specific transcription, the downstream sequences in the UV5 or ps strains are also necessary. The experiment shown allows only a single round of transcription from productive compexes (see Legend). Since the ultimate level of transcription attainable (Table I) is nearly identical for the four competent promoters, recombination with the L305 mutation has at most a minimal effect on the ability of RNA polymerase to ultimately form a stable complex and initiate at promoters containing the UV5 or ps sequences.
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